Use merged child meshes for more reliable CSG operations

[tracygardner]

Motivation

• CSG operations frequently fail on composite GLTF/GLB nodes that have no geometry on the parent but multiple children with geometry. The change aims to reliably prepare those meshes for CSG.
• Temporary merged meshes should be tracked and cleaned up so they don't pollute the scene or leak memory.
• Ensure result meshes inherit appropriate material/transform information from a meaningful reference mesh when children carry materials.

Description

• Enhanced prepareMeshForCSG to merge child meshes into a single temporary mesh for CSG use and tag it with metadata.csgPrepared and metadata.csgSourceName.
• Updated mergeMeshes, intersectMeshes, createHull, subtractMeshes, subtractMeshesMerge, and subtractMeshesIndividual to use prepareMeshForCSG for inputs, to select a material-bearing referenceMesh (via flock._findFirstDescendantWithMaterial) and to collect tempMeshes created for CSG.
• Added explicit disposal of temporary prepared meshes (tempMeshes) and conditional disposal of prepared source meshes (metadata.csgPrepared) after CSG operations complete or fail.
• Preserved original behaviour for applying transforms/materials by using a proper referenceMesh and copying properties via applyResultMeshProperties while avoiding leaking cloned prep meshes.

Testing

• No automated tests were run for this change.

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Codex Task